Advances in MOSFET technology as well as advances in integrated circuits have made it possible to apply class D amplifiers to audio applications. Class D amplifiers are significantly more efficient than class AB amplifiers. The disadvantages are higher part count, cost, electromagnetic interference, and poor performance. With increased integration and the introduction of sophisticated control integrated circuits these disadvantages are becoming less pronounced. In the near future, class D amplifiers will replace class AB amplifiers in many applications. Class D amplifiers already have a clear advantage in high power applications. As the cost and component count of these amplifiers fall, class D amplifiers will be able to complete with class AB amplifiers in low and medium power applications.
To overcome the poor performance of class D amplifiers, others have suggested a self oscillating variable frequency modulator as shown in FIG. 1. An integrator 10 has an audio input over an input resistor R.sub.IN. It has a digital feedback input A over resistor R.sub.DFB, and an analog feedback at input B over resistor R.sub.AFB. The respective analog and digital feedback signals A, B, are taken from the output of the bridge circuit 20 and the low-pass filter that comprises the inductor L and capacitor C.sub.LP. For purposes of understanding, let us simply focus on the digital output A and assume that there is no audio input. In this case, the output at point A is a square wave with a 50% duty cycle. When the square wave is high, current flows through R.sub.DFB into the summing junction of the integrator 10. Its output ramps down until it reaches the negative threshold of the comparator 12. R1 and R2 are used to add hysteresis to the comparator 12. These resistors can be used to adjust the comparator positive and negative thresholds. When the output of the comparator 12 goes low, the upper FET 22 turns off and after a short delay the lower FET 24 turns on. The square wave goes low, and current now flows out of the integrator 10 summing junction through R.sub.DFB. The output of the integrator 10 reverses and ramps up until it reaches the positive threshold of the comparator 12. This signals the lower FET 24 to turn off and after a short delay the upper FET 22 turns on. The square wave goes high and the cycle continues. With no audio signal, the output at A is a 50% square wave, and the output of the integrator 10 is a triangle wave.
Now consider the case when an audio signal is applied. Assuming that the audio signal is positive, then current flows through R.sub.IN into the integrator summing junction. Current also flows through R.sub.AFB out of the summing junction (negative feedback). The net contribution of the audio signal to the integrator summing junction current is I.sub.RIN -I.sub.RAFB. When the upper FET 22 is on, the currents I.sub.DFB and (I.sub.RIN -I.sub.RAFB) are both into the summing junction. This speeds up the ramp at the output of the integrator 10. When the lower FET 24 is on, the current through I.sub.DFB reverses and the two current now are in opposite directions. This slows the ramp down. A similar analysis can be applied to the case where the input signal is negative.
Since the hysteresis built into the comparator 12 is constant, the slope of the positive and negative ramps directly effects the positive and negative pulse widths, and therefore the duty cycle and frequency of the comparator output. At the higher positive audio input voltages, the audio output becomes negative and the on time of the high side switch becomes negligible compared to the on time period of the low side switch. The width of the low side pulse is roughly proportional to the output voltage and primarily sets the loop frequency.
A disadvantage of the self-oscillating circuit in FIG. 1 is shown in FIG. 2. There is that shown a plot of the switching frequency of the modulator as a function of the output power. The switching frequency varies inversely with the output power. Thus, at high power output, the switching frequency falls. As the switching frequency falls into a range that approaches the frequency of the audio input, distortion artifacts appear. They are due to an inadequate sampling rate at the reduced frequency. As the switching frequency transverses the audio spectrum, the low pass filter can no longer attenuate the carrier. This energy can damage tweeters.
Thus, it is necessary to keep the switching frequency well above audio frequency. In the past, this has been accomplished by substantially over-designing the amplifier so that its maximum power output is as much as twice the desired power. In other words, to achieve a non-distorted output at 250 watts of power, the prior art class D amplifier is designed to have a maximum power output of as much as 500 watts. From a study of the graph in FIG. 2, it can be seen that as maximum power is increased, the switching frequency for any given output power tends to decrease. If the class D amplifier has a sufficiently high enough power output, there is little or no danger that the switching frequency will fall into the audio spectrum, i.e., at or about 20 kHz or less. Of course, the disadvantage of such an amplifier is that its component parts are substantially oversized in order to accommodate the high output power.
Another solution to the problem has been to limit the audio input. Others have used pre-amplifiers connected between the audio signal and the input to the integrator. The pre-amplifier attenuates the audio input signal in order to make sure that the overall output of the class D amplifier remains substantially less than its maximum power output. This solution has the drawback of reducing the maximum output power. Furthermore, this approach will not work if the bus voltage is low, because the carrier frequency will be much lower at a specified output power. Unless some provision is made to roll the gain off slowly, the input clamping circuit will produce high order harmonics and the music will sound harsh.
As a result of these deficiencies in prior art class D amplifiers, there is a need for an amplifier that is not only self-oscillating but also prevents the switching frequency from falling into the audio range without oversizing the devices and the overall power for the amplifier.